Hydraulic percussive implement

ABSTRACT

The striker piston of a hydraulic impacting implement, such as a road breaker, has first and second effective piston areas presented respectively to first and second chambers in a casing. The first piston area is greater than the second piston area and the striker piston has a free end which projects from the second chamber so that it can strike a tool held in a tool holder towards the end of the working stroke of the striker piston. A collar is formed on that portion of the striker piston which reciprocates in the second chamber and, should the striker piston overshoot, this collar will enter a dashpot cavity. The collar has an effective area presented towards the dashpot cavity which is larger than the second piston area of the striker piston.

United States Patent [1 Reynolds et a1.

[ HYDRAULIC PERCUSSIVE IMPLEMENT [75] Inventors: Lionel Arthur Reynolds, Painswick near Stroud; David Richard James, l-lasfield, both of England [73] Assignee: A. F. Hydraulics Limited, England [22] Filed: May 1, 1972 21 Appl. No.: 249,223

- 1 1 June 3, 1975 3,713,367 1/1973 Butterworth 91/300 X Primary Examiner-Frank L. Abbott Assistant Examiner-William F. Pate, I11 Attorney, Agent, or FirmCushman, Darby & Cushman 5 7 1 ABSTRACT The striker piston of a hydraulic impacting implement, such as a road breaker, has first and second effective piston areas presented respectively to first and second chambers in a casing. The first piston area is greater than the second piston area and the striker piston has a free end which projects from the second chamber so that it can strike a tool held in a tool holder towards the end of the working stroke of the striker piston. A collar is formed on that portion of the striker piston which reciprocates in the second chamber and, should the striker piston overshoot, this collar will enter a dashpot cavity. The collar has an effective area presented towards the dashpot cavity which is larger than the second piston area of the striker piston.

26 Claims, 9 Drawing Figures HYDRAULIC PERCUSSIVE IMPLEMENT This invention relates to hydraulically operated percussive implements of the type having a piston element presenting opposed effective piston areas respectively to two chambers the first piston area being relatively large and capable of urging the piston to execute a working stroke on application of pressurised fluid to the first chamber, to which its area is presented, and the second piston area being relatively small and capable of urging the piston to execute a recuperation stroke by virtue of pressurised fluid contained in the second chamber to which its area is presented, on reduction of the pressure in the first chamber.

The pressure in the first chamber may be a fluctuating pressure generated as such at a remote pumping point, or alternatively an intermittent pressure generated at the implement by an automatic switching valve mounted on or near the implement and supplied with fluid at a substantially constant pressure generated at the remote pumping point.

The pressure in the second chamber may be provided by fluid spring means constituted by or associated with the second chamber, or alternatively the said substantially constant pressure generated at the remote pumping point may be constantly supplied to the second chamber.

The piston element has a rod portion passing through a seal in the end wall of the second chamber and the end of the rod portion, or a hammer block attached thereto strikes, towards the end of the working stroke, a tool supported in a casing attached to or integral with the casing defining the two chambers.

It is preferred to use a high pressure acting on the first piston area, with a correspondingly small flow, to minimise pumping losses in the conduit leading from the remote pumping point to the implement. Therefore only a limited area is required for the first piston area. Nevertheless the area required for the second piston area is substantially smaller still since a large part of the energy required for the recuperation stroke is provided by the rebound from the tool after impact at the end of the working stroke and any excess fluid displacement in the second chamber wastes energy whether a fluid spring is used or a constant pressure fluid supply is connected to the second chamber.

In any event, where a constant pressure is intermittently applied to the first chamber and the same pressure is constantly applied to the second chamber the force acting on the first piston area must be able to overcome the force acting on the second piston area by a considerable margin to enable the working stroke to take place and this dictates a considerably smaller sec ond piston area than the first piston area. When the tool breaks through that upon which it has been working or when the operator lifts the implement away therefrom, (hereafter referred to as break-through or lift-off respectively), the energy of the working stroke is no longer expended by impact upon the tool and damping means must be provided to absorb this energy and prevent it from damaging the casing of the implement.

It is known to solve this problem by providing a dash pot cavity which is entered, when the piston element overshoots, by a part of the piston element where it changes in cross-sectional area, to provide an annular area which bears against fluid trapped in the clash pot cavity. It has been the predominant practice in the past to use for this purpose the annular area of the piston which constitutes the second piston area and the area necessary to provide adequate overshsoot damping to the piston element is almost invariably much larger than the optimum area for providing the recuperation stroke of the piston.

It is however also known to provide a separate mass, as part of the piston, which oscillates in a separate chamber located between the first chamber and the second chamber, this separate chamber having a damping cavity entered by the separate mass when the piston exceeds the normal working excursion. The length of this separate chamber involves a substantial addition to the total length of the implement, which is a serious disadvantage and may well rule out its use in a hand-held implement.

According to the invention there is provided an hydraulically operated percussive implement having a piston element presenting opposed effective piston areas respectively to two chambers, the first piston area being relatively large and capable of urging the piston to execute a working stroke on application of pressurised fluid to the first chamber, to which its area is presented and the second piston area being relatively small and capable of urging the piston to execute a recuperation stroke by virtue of pressurised fluid contained in the second chamber to which its area is presented, in reduction of the pressure in the first chamber the piston having an enlargement which enters a damping chamber when the normal working stroke is exceeded characterised in that the said enlargement comprising a land or collar on that part of the piston rod which extends through the second chamber, and a dash pot cavity being provided in the second chamber adjacent to the exit seal therefrom, the land or collar presenting to the dash pot cavity an effective dash pot piston area substantially exceeding that of the second piston area acted upon by the pressure in the second chamber to provide the recuperation stroke of the piston element, the second chamber being formed so as to provide free passage of fluid past the outer margin of the land or collar except when the same has entered a dash pot cavity.

In this way, with a minimum addition to the overall length of the implement, the three piston areas, namely the first area upon which the pressure in the first chamber acts to provide the working stroke, the second area upon which the pressure in the second chamber acts to produce the recuperation stroke and the piston area of the land or collar which co-operates with the dash pot cavity to damp piston overshoot at the end of the working stroke, can be chosen independently of one another and can thus be individually optimised for their respective functions.

In a preferred form of the invention the piston is bored out to receive a tube fixed to the body of the implement so as to provide two effective piston areas, one being the tube itself in co-operation with the said bore in the piston and constituting the first piston area and the other being the annular face of the piston bounded by the tube and a cylinder surrounding the piston and constituting a third piston area operating in a third chamber, the volumes of the cylinders in which the two piston areas operate expanding and contracting in unison and in the opposite sense to the variation on the volume of the cylinder in which the second piston area operates.

This form of the invention may comprise a valve adapted alternatively to admit pressurised fluid to the first chamber or permit fluid to escape from the first chamber to a low pressure zone, an actuating chamber for the valve and a connection therefrom to a port which is uncovered by the said annular face when the same approaches the end of its stroke out of the third chamber to put the valve actuating chamber into communication with the third chamber, a port being uncovered by a piston face defining the second piston area when the piston approaches the end of its recuperation stroke to put the valve actuating chamber into communication with the second chamber, and one way valve means oriented to permit escape of fluid from the third chamber whereby the pressure in the third chamber is reduced as the said annular face withdraws therefrom, whereby the valve actuating chamber is subjected alternately to a low pressure and a high pressure to operate the valve to effect respectively the escape of fluid from the first chamber and the admission of pressurised fluid to the first chamber.

It has been indicated above that a large part of the energy required for the recuperation stroke is provided by the rebound from the tool after impact at the end of the working stroke. It is nevertheless necessary to make supplementary provision for recuperation of the piston (in the form of the second piston area) to ensure correct operation of the implement when starting from rest or on break through or lift-off, since the rebound off the tool is not then available.

There is therefore an excess of energy for recuperation of the piston during normal working, and this tends to make the oscillation frequency of the piston rise unduly.

According to yet another of its features therefore the invention comprises an implement of the type described with means for causing fluid to be displaced through a restrictor at least during the latter part of the recuperation stroke.

In its most basic form this means comprises a restrictor in the low pressure fluid conduit leading from the first chamber. If this restrictor, (hereinafter called the exhaust restrictor"), is located between the valve and the first chamber it is desirable for it to be by-passed by a one-way valve to permit unrestricted flow into the first chamber but preferably the exhaust restrictor is located between the valve and the main exhaust connection from the implement. The performance of implements of this type is apt to be affected by the viscosity of the hydraulic fluid so as to perform inadequately when the viscosity of the fluid is high (e.g., on initially starting up in cold weather) or to operate at an excessively high impacting rate when the viscosity is low, (e.g., when the hydraulic fluid is hot in hot weather and/or due to prolonged working).

These effects may be mitigated by using, for the exhaust restrictor, a viscosity-compensating restrictor or an adjustable restrictor which may be manually operable by a control which may be labelled press when cold or any other appropriate legend.

In a more sophisticated arrangement for controlling the impacting rate the land or collar previously referred to is caused, towards the end of the recuperation stroke, to enter a second dash pot cavity in the second chamber remote from the said dash pot cavity adjacent to the exit seal.

The invention will be more readily understood from the following description of embodiments thereof illustrated in the accompanying drawings in which:

FIG. 1 is a diagram of an embodiment of the invention in its most basic form;

FIG. 2 is a longitudinal section of a second embodiment of the invention;

FIG. 3 is a longitudinal section, to an enlarged scale, of a part of the second embodiment;

FIG. 4 is a longitudinal section. to an enlarged scale, of another part of the second embodiment;

FIG. 5 is a longitudinal section of a third embodiment of the invention;

FIGS. 6 and 7 are longitudinal sections of the third embodiment of the invention at different points in the operating cycle;

FIG. 8 is a longitudinal section, to an enlarged scale of a part of the third embodiment of the invention, in a slightly different form; and

FIG. 9 is a longitudinal section, to an enlarged scale, of a part of the third embodiment of the invention, in a slightly different form.

In FIG. 1, a piston element, generally indicated at 1, consists of a piston head 2 and a piston rod 3, surrounded by a casing generally indicated at 4, which encloses a cavity generally indicated at 5, to receive the piston element.

The cavity has a part 6 bored out to receive piston head 2 with a sliding fit over a length which may be a little longer than the length of piston head 2 plus the length of the stroke to be executed by piston element 1.

Cavity 5 has a further part 7 bored out to a larger diameter than cavity part 6 over most of its length but having a dash pot section 8 of reduced internal diameter at its end remote from cavity part 6.

The end of cavity part 6 to the left of piston head 2 constitutes the first chamber and the end of cavity part 6 to the right of piston head 2, together with cavity part 7, constitute the second chamber.

Piston rod 3 carries a land or collar 9 of substantially larger overall diameter than piston head 2 and the internal diameter of dash pot section 8 is such as to receive land or collar 9 with a relatively loose fit so that the clearance between the two may constitute a resistive escape path for fluid trapped in the dash pot.

A port 10 leads into the end of cavity part 6 to the left of piston head 2, to permit admission into and escape from the first chamber, of pressurised fluid from a source diagrammatically indicated at 11 by a symbol indicating fluctuating pressure which may represent a remote source of fluctuating pressure or a commutating valve on the implement.

A port 12 may be provided in cavity part 7 to permit the application to the second chamber of a constant high fluid pressure from a source diagrammatically indicated at 13 by a symbol which may represent a remote source of constant pressure. Where the second chamber takes the form of a fluid spring, port 12 will be omitted.

The left hand end face 14 of piston head 2 constitutes the first piston area. The second piston area is constituted by the difference between the area of piston head 2 and the area of the piston rod 3 where it passes through a dividing wall 17 of casing 4. The right hand annular end face 16 of land or collar 9 represents the dash pot piston area.

The right hand end of piston element 1, either directly or through a hammer block integral with or attached thereto, strikes a tool not shown.

The dividing wall 17 is bored out to accommodate piston rod 3 with a sealing fit.

In operation, when high pressure is applied at port piston element 1 is driven to the right, (in a working stroke), against the force due to the pressure in cavity 5 to the right of piston head 2 acting on the smaller second piston area and when high pressure is removed from port 10, the force on the second piston area is able to drive piston element to the left, (in a recuperation stroke).

If the piston element overshoots at the end of the working stroke due to break through of the tool or liftoff of the implement by the operator, land or collar 9 enters dash pot cavity 8 and traps fluid therein which escapes relatively slowly through the clearance around land or collar 9 or through any other resistive escape path provided.

Generally land or collar 9 will not enter the dash pot cavity 8 but should it do so, due for instance to break through of the tool then land or collar 9 would only slowly be released from dash pot cavity 8 since the pressure in the second chamber, acting on the larger annular area of land or collar 9 would, on any leftwards movement of piston 1, result in a rightwards force on piston element 1, which would exceed the leftwards force due to the same pressure acting on the smaller second piston area, until fluid could flow through the clearance around land or collar 9 to equalise the pressure on the two sides of land or collar 9.

To avoid this the right hand end of dash pot cavity 8 preferably communicates, via a passage 18 containing a one-way valve 19, with cavity part 7. When the pressure in dash pot cavity 8 is raised on entry into it of land or collar 9, valve 19 is closed but should land or collar 9 fail to emerge from dash pot cavity 8, the former will move to a position at which the pressure in the latter is equal to the pressure in cavity part 7 so that valve 19 will open to permit fluid to flow into dash pot cavity 8 as the pressure in the second chamber acting on the second piston area moves piston element 1 to the left.

FIGS. 2, 3 and 4 show the rather more sophisticated second embodiment of the invention. The reference numerals used in FIG. 1 are repeated for corresponding items.

The piston element 1 is externally of very similar form to that of FIG. 1 but the diameter, at least at the piston head, is larger and the upper end of piston head 2 does not form the first piston area as in FIG. 1.

Instead the piston element 1 is bored out at 20 to receive a tube 21 connected to the upper end of casing 4 and held in position by a casing head 22. The lower end wall of bore 20 constitutes the first piston area and is denoted 14 as in FIG. 1.

The upper annular end face 23 of piston head 2 rides up and down in the bore 6 above piston head 2 which forms an annular third chamber denoted 24. End face 23 may be regarded as an idle annulus so far as the execution of working strokes and recuperation strokes is concerned. It is nevertheless harnessed to a useful function, in this embodiment of the invention, in connection with the operation of a switching valve 25, to be described later.

At the top of chamber 24 there is a one-way valve 26 (better seen in FIG. 4) comprising a shallow cup 27 having holes 28 spaced around it, these holes being normally obscured by an annular spring washer 29 held in place by its inner margin being trapped between cup 27 and an end flange on tube 21. The whole assembly is held in position by the casing head 22, secured to casing 4 by bolts (not shown) or any other convenient means.

Chamber 24 tends to receive a certain relatively small amount of fluid by leakage from the second chamber 7 past the outside of piston head 2 and from the first chamber 20 past the outer surface of tube 2]. This is expelled through one-way valve 26 when annular piston face 23 rises during the recuperation stroke of the piston element 1. When piston element 1 descends valve 26 closes and a partial vacuum is caused in chamber 24.

Dividing wall 17 of FIG. 1 takes the form ofa detachable sleeve 30 backed by a further sleeve 31 held in place by a shoulder 32 in a lower casing 33. Sleeve 30 provides a high pressure seal of conventional form around piston rod 3. The upper part of sleeve 30 defines dash pot cavity 8 as can be more clearly seen in FIG. 3. Sleeve 31 carries a low pressure seal and a wiper seal.

The passage 18 of FIG. I is represented by a number of axial bores, (also denoted 18), through the sleeve 30, leading from chamber part 7 to a recess 34 undercut outwardly from the base of dash pot cavity 8.

The valve 19 of FIG. 1 is represented by an annular ring normally resting clear of the lower ends of bores 18, upon a series of pimples 35 spaced around the lower surface of recess 34.

When land or collar 9 passes into dash pot cavity 8, it initially displaces fluid upwardly through the bores 18 but the flow between the upper surface of recess 34 and the upper surface of annular ring 19 draws the latter upwards against the former due to the Bernoulli effect, whereafter fluid can only be displaced past the clearance around land or collar 9, past the main seal between sleeve 30 and piston rod 3, or through any other leakage path that may exist.

When piston element 1 comes to rest after land or collar 9 has entered dash pot cavity 8, ring 19 will fall away from the upper surface of recess 34 permitting fluid to flow from cavity part 7, via bores 18 into recess 34 so that the pressures on both sides of land or collar 9 equalise, permitting the force on second piston area to raise piston element 1 in the normal way.

Casing 33 is fixed to casing 4 by any convenient means (not shown) and the former provides a socket for a tool 36.

The valve 25 and its mode of operation will now be described.

A two-land spool 37 rides in a bore having two side ports 38 and 39 and a central port 40 leading via port 10 to the first chamber 20 through tube 21.

The end of spool 37 nearest to port 38 has a smaller diameter spigot 41 housed in a cavity and the other end of spool 37 has a large diameter spigot 42 housed in a cavity.

A high pressure fluid connection 43 is connected via conduit 44 to port 38, to a port 45 leading to the interior of chamber part 7, and to the cavity containing spigot 41.

A low pressure fluid connection 46 is connected via a conduit 47, to port 39 and to a leakage recovery port 48 between sleeves 30 and 31.

Conduits 44 and 47 are also connected to ports of a stop-start valve 49 operated by a hand lever 50 pivotted on casing head 22. When the implement is operating the high pressure fluid bears constantly upon the end face of small diameter spigot 41.

The cavity containing large diameter spigot 42 is connected via a conduit 51 to a port 52, (which may be an annular groove), in the wall of bore 6 at a position such that it is cleared by the face 23 of piston head 2 when the latter approaches the lower end of its stroke to put the cavity containing spigot 42 into communication with chamber 24 which is then at low pressure so that spool 37 moves to the left, driven by the high pressure acting on spigot 41 to put the first chamber 20 into communication with the low pressure connection 46, since the neck of spool 37 bridges ports 39 and 40, with port 38 closed off, when spool 37 is in the left hand position.

Piston element 1 then rises because of the pressure in chamber part 7 acting on second piston area.

A digression is necessary at this point to describe the upper dash pot which may be provided, as previously explained, to enable the oscillation frequency of piston l to be controlled. Above land or collar 9 the diameter of piston rod 3 is enlarged, at 55, for a short distance, to the same diameter as that of bore 6.

An upper dash pot cavity 54 is provided at the top of chamber 7 and this is entered by land or collar 9 toward the end of the recuperation stroke.

[t is required that port 52 shall be exposed to the high pressure in the second chamber when piston element 1 reaches (of closely approaches) the upper end of the stroke. This is ensured by providing an annular lower edge to the piston head 2. This edge 15 may be said to delimit the second piston area but its annular area is not necessarily equal to the second piston area since the piston element is preferably necked in immediately below edge 15 to improve fluid flow conditions around this part of piston element 1. The actual dimensions of the second piston area are determined by the difference between the cross-sectional areas of piston head 2 and piston rod 3 where it passes through seal sleeve 30.

Enlargement 55 enters bore 6 to seal off dash pot cavity 54 before edge 15 reaches port 52 and a by-pass passage 56 is provided to ensure that fluid from chamber part 7 can reach port 52 when edge 15 reaches it. Enlargement 55 prevents the escape of fluid, trapped in dash pot cavity 54 when it is entered by land or collar 9, from escaping via passage 56, or passage 51 when port 52 has been uncovered by edge 15.

In many applications the energy to be absorbed by the upper dash pot is small enough for a relatively large clearance to be provided between dash pot cavity 54 and land or collar 9 so that the pressure on both faces of the latter rapidly equalises when the piston element 1 comes to rest and fluid is readily drawn into cavity 54 through this clearance when piston element 1 starts its downward working stroke. If more severe damping of the recuperation stroke should be required, however dash pot cavity 54 may be equipped similarly to dash pot cavity 8, with a non-return valve. This is shown in FIG. 4 where the parts of this upper dash pot, are given the same reference numerals as those given to the parts of the valve of dash pot cavity 8 but with the numerals primed. As the area of the lower face of land or collar 9 acts as a piston area when it enters cavity 54 the force due to the pressure in chamber 7 acting on this area would oppose the force acting on the area of piston area 14 (which may be smaller). Without the valve 35' and ports 18', this would persist until such time as the fluid had leaked past the clearance around the periphery of land or collar 9 to balance the forces on both faces thereof. Furthermore, without valve 35' and ports 18' the initial downward velocity of piston I would be limited to the rate at which such leakage around land or collar 9 could maintain this balance of forces.

There is a residual upward force on land or collar 9 so long as enlargement 55 remains in bore 6 due to the upper annular face of land or collar 9 being smaller than its lower annular face 16 but in fact this is negligible because the difference in diameter between bore 6 (which governs the diameter of enlargement 55), and the diameter of piston rod 3 where it passes through seal 30 is greatly exaggerated in the drawings, for the sake of clarity, and may in practice be of the order of 0.0l inches.

Dash pot cavity 54 and land or collar 9 are proportioned to limit the oscillation frequency of piston l to an acceptable maximum and though an adjustable leakage path may be provided in an experimental prototype, the optimum degree of leakage, once ascertained, is built permanently into production models.

It has previously been indicated that the upper dash pot 54 may not be necessary and that a restrictor in the exhaust passage from the first chamber may be employed to regulate the impacting rate of the implement. Such a restrictor would preferably be inserted in the connection 47 and is diagrammatically indicated by an X in FIG. 2 and given the reference numeral 67.

To resume the description of the valve 25 and its means of operation; towards the end of the upward recuperation stroke of piston element 1, the edge 15 of the piston head 2 passes port 52 exposing the latter, via by-pass passage 56, to the high pressure within the second chamber so that the force acting on spigot 42, by reason of its larger diameter, overrides the force acting on spigot 41, (both spigots being now subjected to the same pressure) so that spool 37 is driven to the right closing off port 39 and bridging ports 38 and 40 to apply the pressure from connection 43 to the first chamber 20. Since piston area 14 is substantially larger than the second piston area, piston element 1 is now forced downwards upon tool 36 to execute the working stroke. As soon as the edge 15 of piston head 2 has passed over port 52 the cavity containing spigot 42 is closed and hydraulically locked so that spool 37 is held over to the right until piston face 23 uncovers port 52 enabling the cavity containing spigot 42 to discharge into chamber 24 which is at this time below atmospheric pressure. Spool 37 then again moves to the left.

The oil displaced by spigot 42 into chamber 24 is discharged with other fluid leaking into it, to low pressure connection 46 via valve 26 and conduit 47, on the upstroke of piston l.

A hydraulic accumulator may be connected to the high pressure fluid connection 43 to maintain a substantially steady operating pressure. It operates by storing energy during the recuperation stroke which makes for improved efficiency and also reduces pressure surges in the supply pipe.

The spool of on-off valve 49 is spring loaded upwardly to a position in which it uncovers a port connected to conduit 44 and puts it into communication with another port connected to conduit 47 so that the high pressure fluid at connection 43 is short-circuited to low pressure connection 46. The implement is thus disabled.

The movable handle 50 rests above one of the main handles 53 of the implement and is automatically lowered on grasping that main handle, to force the spool of on-off valve 49 downwards to the position shown in FIG. 2 where the port connected to high pressure connection 43 is blocked.

On-off valve 49 may. alternatively, operate when handle 50 is released (and allowed to rise), to close off high pressure fluid connection 43 so as to isolate it from conduit 44 and in this case the low pressure connection from valve 49 to low pressure connection 46 may be omitted or confined to the depressurisation of chamber 20. This alternative method of operation of valve 49 is desirable when two or more implements are connected in parallel to a single pump.

Instead of directly isolating high pressure connection 43 from conduit 44, valve 49 may be arranged simply to connect the chamber containing valve spigot 42 directly to the low pressure connection 46. Valve 25 is then held in the exhaust condition characteristic of the recuperation stroke of piston unit 1 but the valve cannot change over to the working stroke conditions. Preferably valve 49 also blocks conduit 44 to prevent chamber 7 from being drained to low pressure by leakage past valve 37.

In another alternative form for the on-off valve 49, the valve, in the of position, blocks the exhaust path, preferably between port 39 of valve 25 and the low pressure connection 46, i.e., at some point along connection 47. This is also the recommended location for the exhaust restrictor 67 and the two functions may be performed by a single valve, for instance a needle valve. This valve could be urged to the closed position by two springs, a strong spring and a weak spring. Depression of lever 50 would compress the strong spring leaving the needle valve under control of the weak spring urging the valve to the closed position in opposition to a force due to the pressure up-stream of the valve closure member, acting on a piston face of the valve spool. With lever 50 thus depressed the valve would be to a certain extent viscosity-compensated and would tend to correct the tendency of the implement to perform inadequately when the viscosity of the working fluid was high (e.g., on starting in cold weather) or to speed up to an unacceptable impacting rate when the viscosity of the working fluid was low (e.g., in hot weather and/or after prolonged use). Skilful use of hand lever 50 would enable the compression of the strong spring to be initiated gradually so that the initial blows of the chisel could be relatively weak, which is sometimes required when the target area for the chisel is to be accurately located.

With forms of valve 49 which block the flow of high pressure fluid through the implement without passing this flow to the low-pressure connection 46 it is advantageous to use, for the remote pressurised fluid source, a pump of the automatic pressure regulating type, for instance an axial piston pump of the swash-plate type having automatic means for adjusting the angle of the swash-plate so as to reduce the displacement of the pistons if the pressure tends to rise due tothe blocking of the flow to the implement, or vice versa if the pressure tends to fall on resumption of the flow to the implement.

Valve spool 37 may be urged to the left by a spring instead of the constant application of high pressure fluid to the cavity housing the smaller diameter spigot 41.

The third embodiment will now be described in relation to FIGS. 5 to 9 in which the reference numeralsl used in FIGS. 1 to 4 for the various items of those Figures will be repeated for corresponding items. FIGS. 5, 6 and 7 are somewhat simplified as compared with FIGS. 2 to 4 and details omitted from FIGS. 5, 6 and 7 for the sake of simplicity are to be considered as included in the third embodiment of the invention.

In this third embodiment the one-way valve 19 associated with the lower dash pot 8 is omitted and land or collar 9 has been somewhat increased in area, the cross-sectional area of the clearance, (denoted 60 in FIG. 8), between land or collar 9 and the outer shell of dash pot 8 being correspondingly increased so as to maintain the desired damping characteristic. When the piston is arrested by this damping, the pressure on both sides of land or collar 9 equalises via the clearance 60. As the piston rises due to the pressure in chamber 7 acting on the second piston area 15, the pressure on the lower side of land or collar 9 is progressively equalised by flow through clearance 60 and this limits the rate at which the piston can rise. This may be an advantage in certain applications since it substantially reduces the oscillation frequency of the piston l on break through or lift-off, thus reducing the build-up of heat in the fluid in chamber 7 due to repeated action of the damper 9/8. To ensure that the pressure on the lower side of land or collar 9 can effectively equalise with that on its upper side, when the land or collar 9 reaches the bottom of its stroke into dash pot 8, the bottom ofthis dash pot is grooved, at 34, and the lower surface of land or collar 9 is chamfered at 61. The second embodiment contains similar features, of course.

An upper recoil dash pot 54 is provided in the third embodiment as with the second embodiment, but the arrangements for operating valve 37 are different from those in the second embodiment, in that the pressure built up in the recoil dash pot 54 is utilised to operate valve spool 37 to the right (see FIG. 2), thus enabling the transfer port 56 (FIGS. 2 and 4) to be dispensed with. This must be achieved without undue reduction of the damping provided by recoil dash pot 54, so that there must be adequate restriction of the flow therefrom to dissipate the excess recuperation energy but this must not be such as to render the operation of valve 37 too sluggish. A compromise between these conflicting requirements is not too difficult however since they both tend to achieve the same end, namely to hold the impacting rate of the implement to an acceptable upper limit.

FIGS. 5, 6, 7 and 9 illustrate two practicable methods of actuating valve 37 from the pressure built up in recoil dash pot 54. Two separate ports 52 and 52, spaced apart along the axis of the implement, replace the single port 52 of FIGS. 2 and 4. The length of piston head 2 is increased by an amount substantially equal to the spacing between ports 52 and 52', or alternatively the piston head length of FIGS. 2 and 4 is retained but the stroke shortened by the same amount. Ports 52 and 52' unite on the way to the chamber containing spigot 42 of valve 37, as shown at 62 in FIG. 5.

At the top of the stroke of piston l, the lower port 52 is uncovered by annular piston edge 15, and at the lower end of the normal operating stroke of piston l the upper port 52' is uncovered. The spigot 55 above land or collar is retained but its clearance from the walls of bore 6 is increased.

When land or collar 9 enters dash pot 54, pressure is built up in the latter and is transmitted to the space surrounding the neck 64 of piston 1, between piston head 2 and spigot 55, via the clearance round the latter to the lower port 52 and thence to the chamber containing spigot 42 of valve 37. To ensure an adequate restriction of the flow from dash pot 54, it is desirable to insert a restrictor in the path of flow between the lower port 52 and the chamber containing spigot 42 of valve 37 which does not unduly delay the operation of the latter. This restrictor, 63, is indicated diagrammatically by an X in FIG. 5.

When the piston starts to descend on introduction of high pressure into the first chamber, (via ports 38 and 40 of valve 37) the force acting on the first piston area has to overcome the pressure in chamber 7 acting on the lower face of land or collar 9 until the pressure can equalise on both sides of the latter by flow via the clearance surrounding the same. This clearance must be adequate to ensure that the working stroke of piston l is not unduly interfered with. Any depression in dash pot 54 is communicated, via the space surrounding spigot 55 and the restrictor 63, to the chamber containing valve spigot 42, and this persists until the progress of piston head 2 has closed off port 52. To ensure that valve 37 is not operated to the left during this period, it may be preferable to insert a non-return valve 65, (shown diagrammatically by a Y in FlG. 5), in port 52, oriented to prevent flow via port 52 out of the chamber containing spigot 42. However this precaution may in practice prove unnecessary since the inertia of valve spool 37 may prevent mis-operation in the brief interval before port 52 is uncovered. It is for this reason that the spigot 55, though strictly speaking it could be dispensed with, is preferably retained in the arrangement of FIGS. 5, 6 and 7 since the restriction it offers to flow into dash pot 54 from port 52, due to depression in dash pot 54 at the beginning of the working stroke, may be just sufficient to prevent mis-operation of valve spool 37 without the non-return valve 65. Any restriction furnished by spigot 55 is in series with restrictor 63, of course, and must not be of a magnitude such that the two restrictions unduly delay the operation of valve 25.

Towards the end of the working stroke of piston 1 the annular face 23 of piston head 2 uncovers the upper port 52' to draw off the fluid from the chamber containing spigot 42, independently of port 52 and any restrictor or non-return valve therein. FIG. 6 illustrates this point in the operating cycle.

FIG. 7 illustrates the state of the implement of the third embodiment which is reached when piston 1 overshoots, on break through or lift-off. Land or collar 9 has entered dash pot 8 and it will be noticed that annular face 23 of piston head 2 has uncovered both of the ports 52 and 52. This is of no consequence because valve spool 37 had previously been operated to the left (to the exhaust condition) when port 52' was uncovered towards the end of the normal working stroke as illustrated in FIG. 6.

FIG. 9 shows a modified form of the third embodiment in which the clearance 66 around spigot 55 is alone relied upon to provide the restrictor for dash pot 54, restrictor 63 being omitted. The fluid escaping via clearance 66 reaches port 52 to operate valve 25. It may still be necessary to retain non-return valve 65 but if, by suitable proportioning the clearance 66, the axial length of port 52 where it enters bore 6 and the characteristics of valve 25, non-return valve 65 can be dispensed with without mis-operation of valve 25, then it is not essential to provide the additional port 52' and the stroke length and piston head length of the second embodiment can be adhered to.

We claim:

1. A hydraulically-operated percussive implement which includes:

a. a casing,

b. first and second chambers within the casing.

c. a striker piston reciprocable within the casing,

d. means for introducing pressurised fluid into said chambers and for controlling the fluid pressures in said chambers,

e. said striker piston having a relatively large first effective piston area which partly bounds the first chamber and is acted upon when pressurized fluid is introduced into the first chamber to cause the striker piston to execute a working stroke,

f. said striker piston also having a relatively small sec ond effective piston area which partly bounds the second chamber is acted upon by pressurized fluid introduced into the second chamber, and when the fluid pressure in the first chamber is reduced, to cause the striker piston to execute a return stroke,

g. said second chamber having a wall at the end thereof remote from the first chamber which is formed with a sealed aperture through which an end portion of the striker piston projects,

h. the casing including tool holding means for locating a tool so as to be struck by the end of the striker piston towards the end of the working stroke thereof,

i. a collar on that portion of the striker piston which passes through the second chamber, said collar having opposing effective surface areas which are normally both exposed to the pressure in the second chamber and being of such dimensions as to permit of passage of fluid in the second chamber around the collar during normal reciprocatory movement of the striker piston,

j. an overshoot dashpot cavity situated at the end of the second chamber remote from the first chamber and positioned so that, in the event that the length of the working stroke should exceed a normal length as a result of removal of external resistance to advance of the tool, the collar will enter the cavity to trap fluid therein,

k. the effective area of the collar presented to the dashpot cavity being substantially in excess of the second effective piston area of the striker piston, and

l. restrictor means provided as the sole path for escape of fluid trapped in the cavity on entry thereinto of the collar.

2. An implement according to claim 1, wherein a bore is formed in the end of the striker piston remote from that intended to strike the tool and a hollow cylindrical member fixed to the casing enters said bore in sealed relationship therewith, the bore and the hollow cylindrical member cooperating to form the first chamber and the end of the bore affording the first effective piston area.

3. An implement according to claim 2, wherein the end of the striker piston remote from that intended to strike the tool is formed as a piston head which is surrounded by, and has sealing relationship with. a cylinder bore in the casing, the piston head having a first annular face which is bounded by the hollow cylindrical member and the cylinder bore constituting a third effective piston area partly bounding a third chamber which is otherwise bounded by the cylinder bore and the outer surface of the hollow cylindrical member.

4. An implement according to claim 3, wherein the the hollow cylindrical member provides a passage for the flow of fluid to and from the first chamber and wherein the volumes of the first and third chambers increase and diminish in unison on reciprocation of the striker piston.

5. An implement according to claim 4, wherein a valve is provided at a location intermediate the first chamber and a low pressure zone, said valve being operable alternatively to a first position in which it admits pressurised fluid to the first chamber and to a second position in which it permits fluid to escape from the first chamber to the low pressure zone.

6. An implement according to claim 5, which includes an actuating chamber for said valve, a port entering the wall of the cylinder bore at a position at which it is obscured by the piston head for the greater part of the working stroke of the striker piston but is uncovered by the first annular face of the piston head towards the end of the working stroke and a connection from the valve actuating chamber to said port whereby the valve actuating chamber can be put into communication with the third chamber towards the end of the working stroke of the striker piston to operate the valve to move it to its second position.

7. An implement according to claim 6, wherein the piston head is formed with a second annular face, said second annular face partly bounding the second chamber and being situated at the end of the piston head remote from the first annular face.

8. An implement according to claim 7, wherein said port is formed in the cylinder bore at a position at which it is obscured by the piston head for the greater part of the return stroke of the striker piston but is uncovered by the second annular face of the piston head towards the end of the return stroke, and a high pressure zone arranged to communicate with the valve actuating chamber via said port towards the end of the return stroke of the striker piston to operate the valve to move it to its first position.

9. An implement according to claim 8, which includes a fluid passage communicating with the third chamber and a one-way valve in said passage arranged to permit escape of fluid from the third chamber during the return stroke of the striker piston to permit reduction of the fluid pressure in the third chamber.

10. An implement according to claim 1, wherein fluid flow restrictor means are provided which are selectively operable to restrict fluid flow from the first chamber but which permit unrestricted flow into the first chamber.

11. An implement according to claim 8, wherein a recuperation dashpot cavity is located in the second chamber at a position such that it is entered by the collar on the striker piston towards the end of the return stroke of the striker piston. whereby fluid compressed in the recuperation dashpot cavity may pass via said port to the valve actuating chamber to operate the valve to move it into its first position and fluid flow restrictor means being included in said access means.

12. An implement according to claim 11, wherein a one-way valve is interposed between said port and the valve actuating chamber, said one-way valve being arranged to permit fluid flow from said port to the valve actuating chamber and to prevent return flow.

13. An implement according to claim 12, wherein a second port enters the cylinder bore. said second port being so positioned that it is uncovered by the first annular face of the piston head at a position in the working stroke of the striker piston before the first annular face reaches the first-mentioned port associated with the oneway valve.

14. An implement according to claim 8, wherein a single port enters the wall of the cylinder bore to provide communication between said cylinder bore and the valve actuating chamber, this single port being uncovered by the first annular face of the piston head towards the end of the working stroke of the striker piston to operate the valve to move it to its second position, said single port also being uncovered by the second annular face of the piston head towards the end of the return stroke of the striker piston to operate the valve to move it to its first position.

15. An implement according to claim 14, wherein a recuperation dashpot cavity is provided in the second chamber at a position such that it is entered by the collar on the striker piston towards the end of the return stroke of the striker piston and wherein a by-pass port provides a fluid flow path between the second chamber and the wall of the cylinder bore at position such that, during the return stroke of the striker piston, said bypass port is uncovered by the second annular face of the piston head before said face uncovers the port leading to the valve actuating chamber, said by-pass port being at such position that it is never uncovered by the first annular face of the piston head during the working stroke of the striker piston nor during any overshoot thereof.

16. An implement according to claim 1, wherein a recuperation dashpot cavity is located in the second chamber at a position such that it is entered by the collar on the striker piston towards the end of the return stroke of the striker piston.

17. An implement according to claim 1, which includes a first duct for connection to a source of pressure flow, a second duct for connection to a region of low pressure and on-off valve means operable to connect the first duct to the second duct to immobilise the implement, said on-off valve means being manually operable to break such connection to set the implement in action.

18. An implement according to claim 1, including a duct for connection to a source of pressurised fluid and on-off valve means operable to block the passage of fluid from said duct to other parts of the implement to immobilise the implement, said on-off valve means being manually operable to unblock said duct to set the implement in action.

19. An implement according to claim 1, which includes a duct for connection to a region of low pressure and on-off valve means operable to block any passage through which fluid may flow from the first chamber to said duct to immobilise the implement, said on-off valve means being manually operable to permit passage of said fluid from said feed chamber to said duct to set the implement in action.

20. An implement according to claim 19, wherein the on-off valve means is operable between a first condition in which it blocks the fluid flow from the first chamber to said duct and a second condition in which such flow passes through restrictor means.

21. A hydraulically-operated percussive implement comprising a casing with tool-holding means, a striker/- piston reciprocable within the casing to execute work ing and return strokes of the striker/piston, and valve means controlling the flow of hydraulic fluid for actuation of the striker/piston, the striker/piston having an annular collar movable in a chamber in the casing in which chamber the hydraulic fluid pressure acts on the striker/piston to produce return strokes thereof, said collar having opposing effective surface areas which are both normally exposed to the pressure in the second chamber, and said chamber having at one end a dashpot cavity entered by said collar over an end portion of the maximum working stroke of the striker/piston whereby trapping of the fluid in said cavity and restricted escape therefrom clamps overtravel of the striker/piston beyond a normal length working stroke thereof.

22. A percussive implement according to claim 21, wherein the striker/piston has an effective piston area within said chamber, on which the fluid pressure acts to produce said return strokes, which effective piston area is materially less than the effective cross-sectional area of said collar.

23. A percussive implement according to claim 22, wherein the striker/piston has, in addition to said effective piston area, a further effective piston area on which fluid pressure acts in a further chamber to produce working strokes of the striker/piston.

24. A percussive implement according to claim 21, wherein the arrangement is such that in operation with a tool held in said tool-holding means the tool is impacted directly by said striker/piston during said working strokes thereof.

25. A hydraulically-operated percussive implement which includes: a casing providing first and second chambers within the casing, a striker piston reciprocable within the casing and having a relatively large first effective piston area which partly bounds the first chamber and is acted upon when pressurised fluid is introduced into the first chamber to cause the striker piston to execute a working stroke, said striker piston also having a relatively small second effective piston area which partly bounds the second chamber and is acted upon by pressurized fluid introduced into the second chamber. when the fluid pressure in the first chamber is reduced, to cause the striker .piston to execute a return stroke; said second chamber having at one end a wall with a sealed aperture through which an end portion of the striker piston projects and the casing includ ing tool holding means for locating a tool so as to be struck by the end of the striker piston towards the end of the working stroke thereof; means for introducing pressurised fluid into said chambers including valve means controlling the flow of fluid to and from said first chamber; a collar on that portion of the striker piston which passes through the second chamber. said collar having opposing effective surface areas which are normally both exposed to the pressure in the second chamher, said collar being of such dimensions as to permit passage of fluid in the second chamber around the collar during normal reciprocatory movement of the striker piston; an overshoot dashpot cavity situated at said one end of the second chamber and positioned so that, in the event that the length of the working stroke should exceed a normal length as a result of removal of external resistance to advance of the tool, the collar will enter the cavity to trap fluid therein; the effective area of the collar presented to the dashpot cavity being substantially in excess of the second effective area of the striker piston; and at least one flow-restrictive passage which interconnects the overshoot dashpot cavity and the second chamber and at least one unrestricted passage which interconnects the lower end of said overshoot dashpot cavity and said second chamber, one way valve means being provided to prevent the flow of fluid from the overshoot dashpot cavity into the second chamber through any such unrestricted passage but to permit fluid flow into said cavity after the collar on the striker piston has come to rest within said cavity.

26. A hydraulically-operated percussive implement comprising a casing with tool-holding means and having first and second chambers, a striker/piston reciprocable within the casing to execute working and return strokes of the striker/piston, and valve means controlling the flow of hydraulic fluid to and from said first chamber for actuation of the striker/piston with pressure in said first chamber producing forward working strokes of the striker/piston, the striker/piston having an annular collar movable in said second chamber in the casing in which chamber the hydraulic fluid pressure acts on the striker/piston to produce return strokes thereof, said collar having opposing effective surface areas which are normally both exposed to the pressure of the second chamber and said second chamber having at one end a dashpot cavity entered by said collar over an end portion of the maximum working stroke of the striker/piston whereby trapping of the fluid in said cavity and restricted escape therefrom damps overtravel of the striker/piston beyond a normal length working stroke thereof. 

1. A hydraulically-operated percussive implement which includes: a. a casing, b. first and second Chambers within the casing, c. a striker piston reciprocable within the casing, d. means for introducing pressurised fluid into said chambers and for controlling the fluid pressures in said chambers, e. said striker piston having a relatively large first effective piston area which partly bounds the first chamber and is acted upon when pressurized fluid is introduced into the first chamber to cause the striker piston to execute a working stroke, f. said striker piston also having a relatively small second effective piston area which partly bounds the second chamber is acted upon by pressurized fluid introduced into the second chamber, and when the fluid pressure in the first chamber is reduced, to cause the striker piston to execute a return stroke, g. said second chamber having a wall at the end thereof remote from the first chamber which is formed with a sealed aperture through which an end portion of the striker piston projects, h. the casing including tool holding means for locating a tool so as to be struck by the end of the striker piston towards the end of the working stroke thereof, i. a collar on that portion of the striker piston which passes through the second chamber, said collar having opposing effective surface areas which are normally both exposed to the pressure in the second chamber and being of such dimensions as to permit of passage of fluid in the second chamber around the collar during normal reciprocatory movement of the striker piston, j. an overshoot dashpot cavity situated at the end of the second chamber remote from the first chamber and positioned so that, in the event that the length of the working stroke should exceed a normal length as a result of removal of external resistance to advance of the tool, the collar will enter the cavity to trap fluid therein, k. the effective area of the collar presented to the dashpot cavity being substantially in excess of the second effective piston area of the striker piston, and l. restrictor means provided as the sole path for escape of fluid trapped in the cavity on entry thereinto of the collar.
 1. A hydraulically-operated percussive implement which includes: a. a casing, b. first and second Chambers within the casing, c. a striker piston reciprocable within the casing, d. means for introducing pressurised fluid into said chambers and for controlling the fluid pressures in said chambers, e. said striker piston having a relatively large first effective piston area which partly bounds the first chamber and is acted upon when pressurized fluid is introduced into the first chamber to cause the striker piston to execute a working stroke, f. said striker piston also having a relatively small second effective piston area which partly bounds the second chamber is acted upon by pressurized fluid introduced into the second chamber, and when the fluid pressure in the first chamber is reduced, to cause the striker piston to execute a return stroke, g. said second chamber having a wall at the end thereof remote from the first chamber which is formed with a sealed aperture through which an end portion of the striker piston projects, h. the casing including tool holding means for locating a tool so as to be struck by the end of the striker piston towards the end of the working stroke thereof, i. a collar on that portion of the striker piston which passes through the second chamber, said collar having opposing effective surface areas which are normally both exposed to the pressure in the second chamber and being of such dimensions as to permit of passage of fluid in the second chamber around the collar during normal reciprocatory movement of the striker piston, j. an overshoot dashpot cavity situated at the end of the second chamber remote from the first chamber and positioned so that, in the event that the length of the working stroke should exceed a normal length as a result of removal of external resistance to advance of the tool, the collar will enter the cavity to trap fluid therein, k. the effective area of the collar presented to the dashpot cavity being substantially in excess of the second effective piston area of the striker piston, and l. restrictor means provided as the sole path for escape of fluid trapped in the cavity on entry thereinto of the collar.
 2. An implement according to claim 1, wherein a bore is formed in the end of the striker piston remote from that intended to strike the tool and a hollow cylindrical member fixed to the casing enters said bore in sealed relationship therewith, the bore and the hollow cylindrical member cooperating to form the first chamber and the end of the bore affording the first effective piston area.
 3. An implement according to claim 2, wherein the end of the striker piston remote from that intended to strike the tool is formed as a piston head which is surrounded by, and has sealing relationship with, a cylinder bore in the casing, the piston head having a first annular face which is bounded by the hollow cylindrical member and the cylinder bore constituting a third effective piston area partly bounding a third chamber which is otherwise bounded by the cylinder bore and the outer surface of the hollow cylindrical member.
 4. An implement according to claim 3, wherein the the hollow cylindrical member provides a passage for the flow of fluid to and from the first chamber and wherein the volumes of the first and third chambers increase and diminish in unison on reciprocation of the striker piston.
 5. An implement according to claim 4, wherein a valve is provided at a location intermediate the first chamber and a low pressure zone, said valve being operable alternatively to a first position in which it admits pressurised fluid to the first chamber and to a second position in which it permits fluid to escape from the first chamber to the low pressure zone.
 6. An implement according to claim 5, which includes an actuating chamber for said valve, a port entering the wall of the cylinder bore at a position at which it is obscured by the piston head for the greater part of the working stroke of the striker piston but is uncovered by the first annular face of the pIston head towards the end of the working stroke and a connection from the valve actuating chamber to said port whereby the valve actuating chamber can be put into communication with the third chamber towards the end of the working stroke of the striker piston to operate the valve to move it to its second position.
 7. An implement according to claim 6, wherein the piston head is formed with a second annular face, said second annular face partly bounding the second chamber and being situated at the end of the piston head remote from the first annular face.
 8. An implement according to claim 7, wherein said port is formed in the cylinder bore at a position at which it is obscured by the piston head for the greater part of the return stroke of the striker piston but is uncovered by the second annular face of the piston head towards the end of the return stroke, and a high pressure zone arranged to communicate with the valve actuating chamber via said port towards the end of the return stroke of the striker piston to operate the valve to move it to its first position.
 9. An implement according to claim 8, which includes a fluid passage communicating with the third chamber and a one-way valve in said passage arranged to permit escape of fluid from the third chamber during the return stroke of the striker piston to permit reduction of the fluid pressure in the third chamber.
 10. An implement according to claim 1, wherein fluid flow restrictor means are provided which are selectively operable to restrict fluid flow from the first chamber but which permit unrestricted flow into the first chamber.
 11. An implement according to claim 8, wherein a recuperation dashpot cavity is located in the second chamber at a position such that it is entered by the collar on the striker piston towards the end of the return stroke of the striker piston, whereby fluid compressed in the recuperation dashpot cavity may pass via said port to the valve actuating chamber to operate the valve to move it into its first position and fluid flow restrictor means being included in said access means.
 12. An implement according to claim 11, wherein a one-way valve is interposed between said port and the valve actuating chamber, said one-way valve being arranged to permit fluid flow from said port to the valve actuating chamber and to prevent return flow.
 13. An implement according to claim 12, wherein a second port enters the cylinder bore, said second port being so positioned that it is uncovered by the first annular face of the piston head at a position in the working stroke of the striker piston before the first annular face reaches the first-mentioned port associated with the one-way valve.
 14. An implement according to claim 8, wherein a single port enters the wall of the cylinder bore to provide communication between said cylinder bore and the valve actuating chamber, this single port being uncovered by the first annular face of the piston head towards the end of the working stroke of the striker piston to operate the valve to move it to its second position, said single port also being uncovered by the second annular face of the piston head towards the end of the return stroke of the striker piston to operate the valve to move it to its first position.
 15. An implement according to claim 14, wherein a recuperation dashpot cavity is provided in the second chamber at a position such that it is entered by the collar on the striker piston towards the end of the return stroke of the striker piston and wherein a by-pass port provides a fluid flow path between the second chamber and the wall of the cylinder bore at position such that, during the return stroke of the striker piston, said by-pass port is uncovered by the second annular face of the piston head before said face uncovers the port leading to the valve actuating chamber, said by-pass port being at such position that it is never uncovered by the first annular face of the piston head during the working stroke of the striker piston nor during any overshoot thereof.
 16. An implement according to claim 1, wherein a recuperation dashpot cavity is located in the second chamber at a position such that it is entered by the collar on the striker piston towards the end of the return stroke of the striker piston.
 17. An implement according to claim 1, which includes a first duct for connection to a source of pressure flow, a second duct for connection to a region of low pressure and on-off valve means operable to connect the first duct to the second duct to immobilise the implement, said on-off valve means being manually operable to break such connection to set the implement in action.
 18. An implement according to claim 1, including a duct for connection to a source of pressurised fluid and on-off valve means operable to block the passage of fluid from said duct to other parts of the implement to immobilise the implement, said on-off valve means being manually operable to unblock said duct to set the implement in action.
 19. An implement according to claim 1, which includes a duct for connection to a region of low pressure and on-off valve means operable to block any passage through which fluid may flow from the first chamber to said duct to immobilise the implement, said on-off valve means being manually operable to permit passage of said fluid from said feed chamber to said duct to set the implement in action.
 20. An implement according to claim 19, wherein the on-off valve means is operable between a first condition in which it blocks the fluid flow from the first chamber to said duct and a second condition in which such flow passes through restrictor means.
 21. A hydraulically-operated percussive implement comprising a casing with tool-holding means, a striker/piston reciprocable within the casing to execute working and return strokes of the striker/piston, and valve means controlling the flow of hydraulic fluid for actuation of the striker/piston, the striker/piston having an annular collar movable in a chamber in the casing in which chamber the hydraulic fluid pressure acts on the striker/piston to produce return strokes thereof, said collar having opposing effective surface areas which are both normally exposed to the pressure in the second chamber, and said chamber having at one end a dashpot cavity entered by said collar over an end portion of the maximum working stroke of the striker/piston whereby trapping of the fluid in said cavity and restricted escape therefrom damps overtravel of the striker/piston beyond a normal length working stroke thereof.
 22. A percussive implement according to claim 21, wherein the striker/piston has an effective piston area within said chamber, on which the fluid pressure acts to produce said return strokes, which effective piston area is materially less than the effective cross-sectional area of said collar.
 23. A percussive implement according to claim 22, wherein the striker/piston has, in addition to said effective piston area, a further effective piston area on which fluid pressure acts in a further chamber to produce working strokes of the striker/piston.
 24. A percussive implement according to claim 21, wherein the arrangement is such that in operation with a tool held in said tool-holding means the tool is impacted directly by said striker/piston during said working strokes thereof.
 25. A hydraulically-operated percussive implement which includes: a casing providing first and second chambers within the casing, a striker piston reciprocable within the casing and having a relatively large first effective piston area which partly bounds the first chamber and is acted upon when pressurised fluid is introduced into the first chamber to cause the striker piston to execute a working stroke, said striker piston also having a relatively small second effective piston area which partly bounds the second chamber and is acted upon by pressurized fluid introduced into the second chamber, when the fluid pressure in the first chamber is redUced, to cause the striker piston to execute a return stroke; said second chamber having at one end a wall with a sealed aperture through which an end portion of the striker piston projects and the casing including tool holding means for locating a tool so as to be struck by the end of the striker piston towards the end of the working stroke thereof; means for introducing pressurised fluid into said chambers including valve means controlling the flow of fluid to and from said first chamber; a collar on that portion of the striker piston which passes through the second chamber, said collar having opposing effective surface areas which are normally both exposed to the pressure in the second chamber, said collar being of such dimensions as to permit passage of fluid in the second chamber around the collar during normal reciprocatory movement of the striker piston; an overshoot dashpot cavity situated at said one end of the second chamber and positioned so that, in the event that the length of the working stroke should exceed a normal length as a result of removal of external resistance to advance of the tool, the collar will enter the cavity to trap fluid therein; the effective area of the collar presented to the dashpot cavity being substantially in excess of the second effective area of the striker piston; and at least one flow-restrictive passage which interconnects the overshoot dashpot cavity and the second chamber and at least one unrestricted passage which interconnects the lower end of said overshoot dashpot cavity and said second chamber, one-way valve means being provided to prevent the flow of fluid from the overshoot dashpot cavity into the second chamber through any such unrestricted passage but to permit fluid flow into said cavity after the collar on the striker piston has come to rest within said cavity. 